Organic EL device (organic electroluminescent device) has been developing as a typical surface illuminator. The organic EL device shown in FIG. 1(a) comprises a light-transmissive substrate 1, a light-transmissive electrode 3, an organic light emitting layer 2 made of an organic EL material, and a light-reflective opposing electrode 10, which are superimposed in this order. When receiving a voltage between the light-transmissive electrode 3 and the opposing electrode 10, the organic EL device emits light from the light emitting layer 2 to be output through the light-transmissive electrode 3 and the light-transmissive substrate 1.
The organic EL device outputs the light which is emitted from the light emitting layer 2 through the light-transmissive electrode 3 and the light-transmissive substrate 1, but may decrease light output due to total reflection of light at an interface between the light-transmissive electrode 3 and the light-transmissive substrate 1. In view of this problem, the organic EL device is provided with a light dispersion layer 5 as shown in FIG. 1 (a), so as to improve light output by suppressing the total reflection of light at the interface between light-transmissive electrode 3 and the light-transmissive substrate 1.
The light-transmissive substrate 1 may be formed with a fine bumpy surface for acting as the light dispersion layer 5 opposite to the light-transmissive electrode 3. Instead, the light-transmissive substrate 1 may be provided at its surface opposite to the light-transmissive substrate 3 with a coating resin layer containing particles, as the light dispersion layer 5. In both cases, the light dispersion layer 5 has a bumpy surface, not allowing the thin light-transmissive electrode 3 to be formed with a uniform thickness. In view of the problem, the surface of the light dispersion layer 5 is provided thereon with a smoothing layer 11 having a smooth surface opposite to the light-transmissive electrode 3 in the organic EL device shown in FIG. 1 (a), allowing the light-transmissive electrode 3 to be formed with a uniform thickness (e.g., Japanese unexamined patent publication 2006-286616).
This organic EL device contains the light dispersion layer 5 and the smoothing layer 11, both of which define a light guiding layer 4 between the light-transmissive substrate 1 and the light-transmissive electrode 3. The light dispersion layer 5 of the light guiding layer 4 in this arrangement disperses the light emitted from the light emitting layer 2 toward other directions for improving light output by suppressing the total reflection of light at an interface between the light-transmissive electrode 3 and light-transmissive substrate 1.
As dispersing light in the light dispersion layer 5 of the light guiding layer 4, the organic EL device enables to increase light output in diagonal directions. But, this organic EL device decreases light output in the front direction, insufficiently increasing an overall light output.
The present invention is accomplished in view of this problem, and has an object to provide an organic EL device which enables to improve the overall light output by suppressing the reduction of light output in the front direction as well as increasing light output in diagonal directions.